Diabetes is one of the leading causes of mortality in the US and more than half of diabetic patients have cardiovascular complications. However, the mechanisms and the critical factors involved in the development of diabetic vascular complications remain poorly understood. Our recent studies found that microvessels in diabetic rats had increased basal permeability and a markedly augmented permeability response to inflammatory mediators. We also found that circulating microparticles (MPs), the small vesicles released from the cell surface upon activation, were significantly elevated in both diabetic patients and diabetic rats. Most importantly, results derived from our newly developed experiments showed that transfusion of diabetic rat plasma into the circulation of a normal rat causes immediate leukocyte adhesion and increased microvessel permeability, and that removal of MPs from the diabetic plasma abolished the effect, indicating an important role of diabetic MPs in mediating microvascular inflammation. Similar observations were replicated when isolated diabetic MPs were directly perfused into a normal individual microvessel. In addition, MP analysis of diabetic patients' plasma showed a positive correlation between the levels of MPs and their vascular complications. These novel preliminary findings led us to hypothesize that the increased levels of MPs under diabetic conditions are not simply the results of vascular cell activation and apoptosis, but are also vectors that disseminate pro- inflammatory and pro-coagulant mediators throughout the vascular system, exacerbating vascular dysfunction. This application aims to test this hypothesis with three specific aims. Aim 1 is to characterize the cellular sources and identify the mechanisms of increased MPs under diabetic conditions. Aim 2 is to investigate the causal relationship between the increased plasma MPs and the inflammatory manifestation of diabetic microvessels and to identify the mechanisms involved in MP- mediated leukocyte adhesion, thrombus formation, and increased microvessel permeability. Aim 3 is to investigate the correlations between the increased MPs and the clinical status of diabetic patients. We will evaluate the potential of using MP analysis to predict the ris or severity of vascular complications, as well as the efficacy of diabetic therapies. Present knowledge about MPs is largely derived from in vitro studies. Our unique experimental approaches that combine flow cytometry analysis of MPs with single vessel perfusion, plasma transfusion, confocal imaging, electron microscopy, and quantitative permeability measurements in intact microvessels, will provide the most needed in vivo mechanisms of diabetic MP- mediated microvascular inflammation. The direct linkage between laboratory animal findings with patient disease conditions assures that the knowledge gained from the proposed studies will provide new insight into the pathogenesis of diabetes-associated vascular dysfunction and benefit targeted therapies.